Surge arresters are used to protect expensive electrical equipment from overvoltages. A common product for this purpose are cylindrical blocks of metal oxide, for example zinc oxide, so-called varistors. These have the property that the resistance is high at low voltage but low at high voltage. When the operating voltage is higher than the varistor is able to resist and exhibit a high resistance, several varistor blocks are connected in series in a stack. To carry large heavy currents through a stack, a sufficient contact pressure must be achieved between the blocks.
U.S. Pat. No. 5,291,366 (CH 682 858) discloses a surge arrester with a stack of zinc-oxide blocks clamped between two end electrodes with the aid of a clamping member consisting of two insulating elements connecting the two end electrodes.
U.S. Pat. No. 5,912,611 (SE 504 075) discloses a surge arrester with a stack of zinc-oxide blocks clamped between two end electrodes with the aid of a clamping member consisting of at least three insulating elements connecting the two end electrodes.
To achieve improved resistance to transversal mechanical influence, a central pivot member is placed between one end electrode and the nearest zinc-oxide block in the stack.
The dimensioning of a surge arrester is critical and since its function as protection for, for example, a transformer implies that it is to carry a large current for a short period, the risk of breakdown can never be completely excluded. This may occur, for example, by ionization and electrical discharges in or around the varistor blocks which, by means of pressure increase caused by gas generation, may burst the casing of the surge arrester.
For this reason, it is not suitable for the casing to be made of a material that may be fragmented at an internal pressure increase, but instead to be made of rubber or a similar material. On the one hand, the casing should be so strong that it may actively counteract that parts of the varistors are thrown out. On the other hand, the casing should be able to permit pressure relief by releasing generated gas without completely bursting the casing.
In U.S. Pat. No. 5,050,032 (SE 516 123), a balance has been struck between the above-mentioned requirements, wherein a varistor stack and compression loops are radially surrounded by a busting-protective bandage of insulating material provided with openings for pressure relief. The bursting-protective bandage may consist of a plurality of tubular rings arranged at a certain axial distance between them. The casing, for example of rubber, is cast on so that the material also fills up the space between the varistor stack and the rings. The bursting-protective bandage may consist of a thermosetting resin with continuously wound glass or aramide fibres and will then have an essentially square shape.
By ring is to be understood, in SE 516 123 and in this application, essentially every closed curve and thus also curves which deviate from a circular shape. Experiences in practice have proved that the embodiment according to said SE 516 123 has several critical parameters. If the rings are too far away from the varistor stack, the volume of the insulant has to be increased, which, of course, increases the cost, but above all it deteriorates the short-circuit performance because the rubber, or corresponding material, inside the rings prevents the arrester from ventilating and a higher pressure is built up. This results in a much more violent short-circuit behaviour. For this reason, the rings should be as close to the stack as possible. On the other hand, the rings must not be in direct contact with the blocks. If there is no gap between the rings and the block, filled with rubber or the like, an exceedingly powerful bursting of the blocks is obtained, the windings being torn off and pieces of the blocks being thrown out.
For natural reasons, the proposed, approximately square winding provides a considerable variation of the distance between the blocks and the rings. To this is to be added the fact that the successive application of several turns compresses the loops and reduces the stress, provides a slack, in the innermost turns. These turns will then hang down towards the stack. See FIG. 2. There is a considerable risk that the “belly” reaches the stack unless winding is performed with a decreasing tensile stress.